Magnesium barium thioaluminate and related phosphor materials

ABSTRACT

A phosphor and a method of deposition. The phosphor comprises a composition of the formula M′ a Ba 1-a M″ 2 M′″ 4′ :RE, where  
     M′ is at least one element selected from magnesium and calcium, M″ is at least one element selected from aluminum, gallium and indium, M′″ is at least one element selected from sulphur, selenium and tellurium, RE is at least one rare earth element, especially europium or cerium, and 0&lt;a&lt;1. Deposition is preferably by dual source electron beam deposition. The phosphor may be annealed. The phosphor provides a high luminosity blue emission that does not require an optical filter to achieve acceptable colour coordinates for the blue sub-pixel element for a full colour thin film or thick film electroluminescent display. The blue sub-pixel pixel performance meets the luminosity and colour temperature specifications for current generation cathode ray tube displays.

FIELD OF THE INVENTION

[0001] The present invention relates to a high luminosity blue phosphor.In particular, the present invention relates to a blue phosphor that maybe used without an optical filter to provide acceptable colourcoordinates for the blue sub-pixel element of a full-colourelectroluminescent display. In preferred embodiments, theelectroluminescent displays employ thick film dielectric layers with ahigh dielectric constant. In embodiments, the phosphor isM′_(a)Ba_(1-a)M″₂M′″₄.RE, where M′ is selected from magnesium andcalcium, M″ is selected from aluminum, gallium and indium, M′″ isselected from sulphur, selenium and tellurium, and RE is a rare earthelement, especially europium and cerium.

BACKGROUND TO THE INVENTION

[0002] Thin film electroluminescent (TFEL) displays are known and aretypically fabricated on glass substrates. Electroluminescent displayswith thin film phosphors employing thick film dielectric layersfabricated on ceramic substrates, as exemplified by U.S. Pat. 5,432,015provide greater luminance and superior reliability.

[0003] A high luminosity full colour electroluminescent display requiresthe use of red, green and blue sub-pixels. Optical filters are needed toachieve the required colour coordinates for each sub-pixel.Consequently, the thin film phosphor materials used for each sub-pixelmust be patterned so that there is minimal attenuation of the emissionspectrum for each colour of pixel by the optical filters. For relativelylow-resolution displays, the required patterning can be achieved bydepositing the phosphor materials through a shadow mask. For displayswith high resolution, however, the shadow mask technique does notprovide adequate accuracy, and photolithographic methods must beemployed. Photolithographic techniques require the deposition ofphotoresist films and the etching or lift-off of portions of thephosphor film to provide the required pattern.

[0004] Deposition and removal of photoresist films and etching orlift-off of phosphor films typically require the use of solventsolutions that contain water or other protic solvents. Some phosphormaterials, for example strontium sulphide are susceptible to hydrolysis,and water and aprotic solvents may degrade the properties of thephosphor materials.

[0005] The deficiencies in phosphor materials are most severe with thephosphors used for blue sub-pixels, and may be compensated for to someextent by increasing the area of the blue sub-pixels relative to thearea of the red and green sub-pixels. However, such a designmodification demands increased performance from the phosphor materialsused for the red and green phosphor materials, and requires the use ofhigher display operating voltages. The higher operating voltagesincrease the power consumption of the display, decrease the reliabilityand increase the cost of operating the electronics of the display.

[0006] Thick film dielectric structures provide superior resistance todielectric breakdown, as well as a reduced operating voltage. Whendeposited on a ceramic substrate, the thick film dielectric structurewill withstand higher processing temperatures than TFEL devices on glasssubstrates. The increased tolerance to higher temperatures facilitatesannealing of the phosphor films at higher temperatures, to improveluminosity. However, even with the enhanced luminosity that is obtained,thick film electroluminescent displays have not achieved the phosphorluminance and colour coordinates needed to be fully competitive withcathode ray tube (CRT) displays. Moreover, recent trends in CRTspecifications are to higher luminance and higher colour temperature.

[0007] Traditionally, cerium-activated strontium sulphide has been thephosphor material of choice for blue electroluminescence. This materialhas a relatively high efficiency of conversion of electrical to opticalenergy, of up to about 1 lumen per watt of input power. However, theemission spectrum of cerium-activated strontium sulphide contains asubstantial green emission in addition to the required blue emission,producing a cyan colour. This necessitates the use of an optical filterto achieve acceptable blue colour coordinates. The filter substantiallyattenuates the luminosity of the phosphor, and it is therefore difficultto achieve adequate display luminosity. It is known that the spectralemission of cerium-activated strontium sulphide phosphor may be shiftedto some degree towards blue by controlling deposition conditions andactivator concentration, but not to an extent required to eliminate theneed for an optical filter.

[0008] Alternate blue phosphor materials have been evaluated. Theseinclude cerium-activated alkaline earth thiogallate compounds, whichgive good blue colour coordinates, but have relatively poor luminosityand stability. Lead-activated calcium sulphide has also been shown toprovide excellent blue colour coordinates when the lead activator isintroduced as a dimer, but this material is subject to degradation ofthe dimer species into isolated activator atoms that provide anultraviolet rather than blue emission. Europium-activated bariumthioaluminate provides excellent blue colour coordinates and higherluminance, but must be annealed at high temperature to achieve thisperformance.

[0009] Improvements in the luminance and emission spectrum of phosphormaterials used for blue sub-pixels in full colour AC electroluminescentdisplays employing thick film dielectric layers with a high dielectricconstant would be useful. The thick film dielectric structure wouldprovide superior resistance to dielectric breakdown as well as a reducedoperating voltage, compared to thin film electroluminescent (TFEL)displays.

SUMMARY OF THE INVENTION

[0010] New phosphor materials for blue sub-pixels have now been found.Such phosphors may be used without optical filters.

[0011] Accordingly, one aspect of the present invention provides aphosphor comprising a composition of the formula

[0012] M′_(a)Ba_(1-a)M″₂M′″_(4′):RE, where

[0013] M′ is at least one element selected from the group consisting ofmagnesium and calcium,

[0014] M″ is at least one element selected from the group consisting ofaluminum, gallium and indium,

[0015] M′″ is at least one element selected from the group consisting ofsulphur, selenium and tellurium,

[0016] RE is at least one rare earth element, and

[0017] 0<a<1.

[0018] In preferred embodiments of the invention, the phosphor has beenannealed.

[0019] In further embodiments, M″ is aluminum, especially with RE beingeuropium and/or M′″ being sulphur. Preferably, M′ is magnesium, M′″ issulphur and RE is europium.

[0020] In additional embodiments, “a” is in the range of 0.4 to 0.8,especially with the phosphor comprising a eutectic composition, or inthe range of 0.7 to 0.8.

[0021] A further aspect of the present invention provides a method forthe preparation of a phosphor on a substrate, said phosphor comprising acomposition of the formula M′_(a)Ba_(1-a)M″₂M′″₄:RE, where

[0022] M′ is at least one element selected from the group consisting ofmagnesium and calcium,

[0023] M″ is at least one element selected from the group consisting ofaluminum, gallium and indium,

[0024] M′″ is at least one element selected from the group consisting ofsulphur, selenium and tellurium,

[0025] RE is at least one rare earth element, and

[0026] 0<a<1,

[0027] said method comprising using a compound of the formula M″₂M′″₃ asa first source in a dual source electron beam evaporation apparatus andusing a mixture of compounds of the formulae M′M′″ and BaM′″ as a secondsource in said apparatus, said compound of the first source and saidcompounds of the second source being in the ratios to provide acomposition of the phosphor and at least one of the first and secondsources including a compound of the formula REM′″, and

[0028] effecting electron beam evaporation from said first and secondsources to a substrate to be coated with said phosphor.

[0029] In preferred embodiments, the phosphor so obtained is annealed.

[0030] In further embodiments, the method provides phosphors as definedabove.

[0031] In other embodiments, the phosphor is annealed at a temperatureof at least about 850° C., or the phosphor is annealed at a temperatureof at least about 600° C.

[0032] In further embodiments, RE is europium in an amount of not morethan 3 atomic percent, base on the amount of M′ and barium, mostpreferably in which REM′″ is europium sulphide. Europium sulphide may bereplaced in whole or in part with europium oxide.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The present invention is illustrated by the embodiments shown inthe drawings, in which:

[0034]FIG. 1 is a schematic representation of a cross-section of anelectroluminescent element;

[0035]FIG. 2 is a schematic representation of a plan view of anelectroluminescent element;

[0036]FIG. 3 is a graphical representation of data from Example V;

[0037]FIG. 4 is a graphical representation of data from Example VI; and

[0038] FIGS. 5-9 are graphical representations of data from Example VII.

DETAILED DESCRIPTION OF THE INVENTION

[0039] The present invention relates to improving the luminance andemission spectrum of phosphor materials used for blue sub-pixels. It isbelieved that the phosphors of the present invention will have a widerange of uses. The phosphors of the invention are particular describedherein with respect to use in thick film electroluminescent displays,but it is believed that the phosphors may also be used in conjunctionwith thin film electroluminescent displays and in other end-uses.

[0040] A preferred embodiment of the invention is to use the phosphorsin full colour AC electroluminescent displays employing thick filmdielectric layers with a high dielectric constant. The preferred thickfilm dielectric structures provide superior resistance to dielectricbreakdown as well as a reduced operating voltage compared to thin filmelectroluminescent (TFEL) displays.

[0041] One aspect of the invention is directed to improving theperformance of a barium thioaluminate phosphor, at a lower annealingtemperature. In some binary compounds formed from two distinct elementsor pseudo-binary compounds, formed from two distinct compounds, acomposition may be formed with a finite concentration of two constituentelements or compounds and with a minimum melting temperature. Inmetallurgy, such a composition is termed the eutectic composition, butthe phenomenon also occurs where the constituent elements or compoundsare ceramics rather than metals. For instance, in the formulation of amanganese-activated zinc germano-silicate, a pseudo-binaryelectroluminescent phosphor may be formed from zinc silicate and zincgermanate. The phosphor obtained has a lower annealing temperature thaneither of its pseudo-binary components.

[0042] The pseudo-binary phosphor material requires acceptable electrontransport properties. The matrix or host material acts as a medium inwhich electrons can be accelerated with minimal probability ofscattering from impurities, lattice defects or grain boundaries. Thismaximizes the probability that energy transfer from the acceleratedelectrons is via impact excitation of the activator species. Light isemitted as the activator atoms return back to their ground or unexcitedstate. While not bound by any theory, the inventors believe thatelectron transport would be optimized in a pseudo-binary phosphormaterial by substituting atoms at lattice sites of one componentcompound with atoms of the other component having the same valencestate. In this manner, electron scattering due to the substitutions ofatoms in the crystal lattice would be minimized and the dominantmechanism for energy transfer from the accelerated electrons would be byimpact excitation of activator atoms resulting in light emission.

[0043] In barium thioaluminate, the above conditions for substitutioncould be met if barium is substituted with another element from Group IIin the Periodic Table of Elements, for example magnesium or calcium. Theconditions may also be met by substituting aluminum with another elementin Group III of the Periodic Table, for example gallium or indium; or ifsulphur were substituted with another element in Group VI of thePeriodic Table, for example selenium or tellurium. The substitutionsmust occur in a manner that avoids causing a substantial change in thecrystal structure of the phosphor material, or causing a second phase toprecipitate within the phosphor material as a result of thesubstitutions. In addition, the activator of the phosphor must besoluble in the host lattice, and consequently the lattice constant ofthe pseudo-binary host material would need to be such that there isadequate dissolution of the activator species.

[0044] Another aspect of the present invention provides a phosphorformed from a composition of the formula M′_(a)Ba_(1-a)M″₂M′″₄:RE, where

[0045] M′ is at least one of magnesium and calcium,

[0046] M″ is at least one of aluminum, gallium and indium,

[0047] M′″ is at least one of sulphur, selenium and tellurium,

[0048] RE is a rare earth element, especially europium and cerium, and

[0049] 0<a<1.

[0050] In preferred embodiments of the invention, each of M′, M″ and M′″is a single element. In particular, M′ is magnesium, M″ is aluminum andM′″ is sulphur. The preferred rare earth element (RE) is europium.

[0051] In embodiments of the invention, the value of “a” is in the rangeof 0.1 to 0.9, preferably in the range of 0.4 to 0.8. In particularlypreferred embodiments of the invention, the value of “a” is selected sothat the composition forms a pseudo-binary composition, with a meltingpoint that is lower than the melting point of corresponding bariumthioaluminate. In further preferred embodiments, the value of “a” is inthe range of 0.5-0.75 or in the range of 0.7-0.8. As exemplifiedhereinafter, values of “a” in the range of 0.4-0.8 are believed toresult in the formation of a eutectic composition and values of “a” inthe range of 0.7-0.8 are believed to result in the formation of a singlephase composition.

[0052] In a particular embodiment, the phosphor is formed from acomposition of a magnesium barium thioaluminate.

[0053] The elemental composition of the phosphor and its activator maybe selected to provide a blue emission spectrum with colour coordinatesacceptable for blue sub-pixels, without the need for an optical filter.

[0054] The phosphor may be in the form of a thin film electroluminescentphosphor.

[0055] It is anticipated that europium activated magnesium bariumthioaluminate will be less susceptible to hydrolysis than ceriumactivated strontium sulphide, thus rendering it easier to pattern usingphotolithographic techniques.

[0056] The preferred method of deposition of the phosphor on thesubstrate is by the use of dual source electron beam deposition. In sucha method, a compound of the formula M″₂M′″₃, where M″ and M′″ aredefined above, is used as the first source in the dual source electronbeam evaporation apparatus. The compound is conveniently in the form ofa pellet. A mixture of compounds of the formulae M′M′″ and BaM′″, whereM′, M″ and M′″ are as defined above, is used as the second source in theapparatus. The mixture of compounds is also conveniently in the form ofa pellet. The various compounds of the first and second sources are inthe ratios required to provide the required composition of the phosphor.It is understood that the ratios in the pellets might differ slightlyfrom those of the composition of the phosphor, to allow for differentrates of evaporation of the various compounds during the depositionprocess, and it is understood that ratios of compositions in pellets mayneed to be adjusted so as to obtain the desired composition in thedeposited film. It is understood that in a dual source electron beamdeposition process, the compounds of the phosphor are used per se andthere are no by-products of the process. The compounds are evaporatedfrom the respective sources and deposited onto the substrate that is tobe coated.

[0057] If the rare earth metal is europium, the compound REM′″ used inthe method to form the phosphor is most preferably europium sulphide. Inembodiments, the amount of europium is up to 3 atomic percent based onthe combined amount of M′, especially magnesium, and barium. Inembodiments in which the amount of europium is up to 3 atomic percent,the europium sulphide may be placed in whole or in part with europiumoxide, especially Eu₂O₃.

[0058] In preferred embodiments of the invention, the phosphor issubjected to an annealing step. The annealing step is carried out afterthe phosphor has been deposited on the substrate on which it is to beused. The annealing step must be at a temperature that is sufficientlylow to prevent melting or degradation of the substrate. However, thetemperature should also be above the temperature at which annealing willoccur and preferably above the temperature at which the depositedcompounds will form a film having a homogeneous composition. Withceramic materials used in electroluminescent displays with thick filmdielectrics, the temperature is at least about 850° C., and at suchtemperature the period of the annealing should be short e.g. 1-2minutes. Longer periods of time may be used at lower temperatures e.g.at temperatures of 600-650° C., the time may be increased to for example10 minutes or longer. Such times will depend on the particular substratebeing used.

[0059] The preferred substrate is a thick film ceramic material, whichare known in the art. In embodiments, the substrate comprises a ceramicsheet, typically alumina, upon which an electrically conductive film,typically gold or a silver alloy, is deposited. A thick film layerconsisting of a ferroelectric material and typically comprising one ormore of lead magnesium niobate titanate, lead zirconate titanate orbarium titanate is deposited on the electrically conductive film. Thephosphor film is deposited on the thick film layer followed by anoptically transparent but electrically conductive film to form thesecond electrode for the resultant sub-pixel.

[0060] Thin film dielectric layers may be deposited on the thick filmlayer to mediate undesirable chemical and physical interactions betweenthe deposited phosphor film and the thick and other underlying layers.Thin film dielectric layers may also be deposited on top of the phosphorfilm prior to deposition of the optically transparent and electricallyconductive film. Such further thin film dielectric layers may becomprised of alumina silicon oxynitride, yttria, hafnia zinc sulphide,barium tantalate, barium titanate, tantalum oxide, aluminum titanate,strontium titanate and the like.

[0061] The present invention is further illustrated by the embodimentshown in FIGS. 1 and 2. FIG. 1 shows a cross-section of anelectroluminescent device utilizing a phosphor of the present invention.FIG. 2 shows a plan view of the electroluminescent device. Theelectroluminescent device, generally indicated by 10, has a substrate 12on which is located row electrode 14. Thick film dielectric 16 has thinfilm dielectric 18 thereon. Thin film dielectric 18 is shown with threepixel columns, referred to as 20, 22 and 24, located thereon. The pixelcolumns contain phosphors to provide the three basic colours viz. red,green and blue. Pixel column 20 has red phosphor 26 located in contactwith thin film dielectric 18. Another thin film dielectric 28 is locatedon red phosphor 26, and column electrode 30 is located on thin filmdielectric 28. Similarly, pixel column 22 has green phosphor 32 on thinfilm dielectric 18, with thin film dielectric 34 and column electrode 36thereon. Pixel column 24 has blue phosphor 38 on thin film dielectric18, with thin film dielectric 40 and column electrode 42 thereon.

[0062] It will be noted that electroluminescent device 10 does not havean optical filter associated with the blue phosphor 42. In FIG. 1, bluephosphor 42 is a phosphor of the invention and is as described herein.

[0063] The phosphor of the present invention provides a high luminosityblue emission that does not require an optical filter in order toachieve improved and acceptable colour coordinates for the bluesub-pixel element of a full colour electroluminescent display. The bluesub-pixel pixel performance is believed to meet the luminosity andcolour temperature specifications for current generation cathode raytube displays

[0064] The present invention is illustrated by the following examples.

EXAMPLE I

[0065] A magnesium barium thioaluminate phosphor film was formed on athick film substrate and annealed at a temperature of about 850° C.

[0066] The phosphor gave a relatively high energy conversion efficiencyand a luminosity of 70 candelas per square meter with unfiltered CIEcolour coordinates of x=0.13 and y=0.10 when operated at 250 Hz at avoltage that was 70 volts above a threshold voltage of 180 volts. Thisluminosity allows for an areal blue luminosity of 30 candelas per squaremeter when 50% of the active area of a pixel is occupied by the bluesub-pixel. Areal blue luminosity is defined as the blue luminosityaveraged over the nominal image area of a display.

EXAMPLE II

[0067] A series of magnesium barium thioaluminate thin film phosphorsmaterials were prepared by blending powders of aluminum sulphide, bariumsulphide, magnesium sulphide and europium sulphide in the desired ratiosand making pressed pellets of the blended powders. In the series ofphosphor materials, the fraction “a” of barium replaced by magnesium inthe formula Mg_(a)Ba_(1-a)AL₂ S₄:Eu was varied in increments of 0.1 overthe range of a=0 to a=0.5. All of the phosphor materials had a nominalconcentration of europium corresponding to 3 atomic percent of the sumof the magnesium and barium concentrations.

[0068] The pellets were placed in an alumina boat and fired in anitrogen atmosphere using a belt furnace, using a temperature profilesuch that the films were subject to a nominal peak temperature of 900°C. for about 7 minutes. The actual sample temperature may have beenlower than 900° C. because of the thermal capacity of the alumina boat.

[0069] The fired pellets with no magnesium i.e. a=0, did not show anyvisual changes following firing. Photoluminescence measurements on thefired pellets showed that this material was not homogenous when viewedunder a magnifying glass, with different areas emitting differentcolours of light.

[0070] In contrast, pellets with a=0.3 showed some shrinkage, therebyindicating sintering following firing. For a=0.5, substantialdeformation of the cylindrical pellet was observed after firing, withsubstantial broadening of the pellet at its base, indicating thatsignificant melting had occurred. For a=0.4, some deformation occurred,but not to the same degree as for a=0.5.

[0071] The photoluminescence properties of the materials containingmagnesium were examined, using a magnifying glass. It was found that allof the samples showed uniform blue emission. This indicates that thematerials were homogeneous at the level of detail resolvable by amagnifying glass, with no evidence of a luminance-generating secondphase precipitate.

[0072] X-ray diffraction analysis of a sample of material with a=0 i.e.with no magnesium, showed the presence of a variety of compounds,including barium thioaluminate, barium sulphide and one or moreadditional phases that might include Ba₂Al₂S₅, Ba₄Al₂S₇ or Ba₅Al₂S₈.

[0073] X-ray diffraction analysis of a sample of material containingmagnesium, with a nominal value of a=0.2 showed a phase with a crystalstructure very similar to barium thioaluminate but with slightly reducedlattice constants. This might be expected as a result of substitution ofsome barium by magnesium. The sample also contained barium sulphide, butin a lower concentration than for the sample without magnesium i.e.where a=0. There was an absence of the additional phases that werepresent in the sample without magnesium.

[0074] X-ray diffraction analysis of a sample of material containingmagnesium, with a nominal value of a=0.5 showed a barium thioaluminatephase similar to that observed with the sample having a=0.2. However,the concentration of barium sulphide had been reduced to about half ofthat of the sample with a=0.2. There was no appreciable presence of theadditional phases observed in the sample with a=0.

[0075] The CIE colour coordinate of the photo-stimulated light emissionfrom the samples containing magnesium was y=0.10, compared to0.13<y<0.14 for the sample not containing magnesium. This is a blueshift in the emission of the magnesium-containing materials, andindicates improved utility as blue phosphors. The reduced meltingtemperature for the magnesium-containing materials indicates that thematerials should be amenable to annealing at lower temperatures than thematerials not containing magnesium, which would make them morecompatible with thick film dielectric substrates.

EXAMPLE III

[0076] Magnesium barium thioaluminate materials of the formulaMg_(a)Ba_(1-a)Al₂S₄:Eu, with a nominal value of a=0.5 and a europiumconcentration equal to 3 atomic percent of the sum of the magnesium andbarium concentrations, were deposited as thin films on thick filmdielectric structures. The deposition method used was dual sourceelectron beam evaporation, in which one source was a pressed pellet ofaluminum sulphide (Al₂S₃) and the other source was a pressed pelletconsisting of a mixture of barium sulphide, magnesium sulphide andeuropium sulphide. The phosphor films were annealed at a nominaltemperature of 850° C. under nitrogen.

[0077] It should be noted that the stated composition for the materialsin this example is for the source materials, and the composition of thedeposited films may vary from these compositions.

[0078] The electroluminescent emission of the resultant phosphor showedthe same blue shift with respect to material not containing magnesium aswas observed with photoluminescence measurements. The luminance of themagnesium-containing phosphor was about 50 cd/m² at a voltage that was50 volts above the threshold voltage of 160 volts for the fabricatedelectroluminescent pixel, and 90 cd/m² at 100 volts above the thresholdvoltage. The excitation frequency was 120 Hz.

[0079] In contrast, the luminance of a barium thioaluminate phosphorfilm without magnesium at the same excitation frequency, in a structurethat was not identical, was about 25 to 30 cd/m² at 50 volts above the190 volt threshold. The luminance was about 40 cd/m² at 70 volts abovethe threshold voltage.

EXAMPLE IV

[0080] The procedure of Example III was repeated, except that europiumoxide (Eu₂O₃) was used instead of europium sulphide. The remainingcompounds were aluminum sulphide, barium sulphide and magnesiumsulphide, as in Example III. The nominal value of a in the formula ofExample III was 0.5. The europium concentration was 3 atomic percent ofthe sum of the magnesium and barium concentrations.

[0081] It was found that the resultant phosphor had the samecharacteristics as the phosphor of Example III that had been formedusing europium sulphide. It is therefore believed that europium sulphidemay be replaced in whole or in part with europium oxide for europiumconcentrations of up to 3 atomic percent.

EXAMPLE V

[0082] This example illustrates the ability to control the elementalcomposition of a magnesium barium thioaluminate phosphor film depositedon a thick film dielectric structure using the method described inExample II. This example also shows the dependence on elementalcomposition of the colour coordinates of the light emission from theresulting electroluminescent element.

[0083] Two source pellets were used to deposit the phosphor film viz.aluminum sulphide and europium doped magnesium-barium sulphide. In thisexample, the deposition rate of aluminum sulphide relative to that forthe europium-doped magnesium-barium sulphide was controlled by varyingthe electron beam power for the aluminum sulphide source pellet. Thedeposition rates of the two source materials were monitored usingindependent rate monitors. The composition of the deposited films wasmeasured using secondary ion mass spectroscopy (SIMS).

[0084]FIG. 3 shows the ratio of aluminum to the combined alkaline earthpeaks i.e. the sum of the magnesium and barium peaks, observed in theSIMS spectrum plotted against the relative atomic deposition rate ratiofor the two source materials. As can be seen, there is a linearrelationship between the two variables, indicating that the ratio ofaluminum to alkaline earth elements in the phosphor film is proportionalto the relative deposition rates of the source materials.

[0085]FIG. 3 also shows the y colour coordinate of theelectroluminescent emission for the phosphor materials as a function ofthe elemental composition. It is evident over the range evaluated thatthe y coordinate is not very sensitive to the elemental composition,although a tendency to a somewhat higher value is noted for the samplewith the lowest aluminum concentration. The observed variation may bedue, however, to other variables inherent in the materials and processesused in the fabrication of the pixel used. Although not shown, the xcolour coordinate and the luminance also did not show any systematicdependence on the elemental composition over the range evaluated.

EXAMPLE VI

[0086] A series of europium-doped barium thioaluminate phosphor films,with thicknesses of approximately 420 nanometers, were electron-beamdeposited using dual aluminum sulphide and europium doped bariumsulphide sources onto clean 2 inch by 2 inch (5 cm by 5 cm) aluminasubstrates and then annealed at a temperature of 800° C. The europiumconcentration was varied so that the atomic ratio of europium to bariumwas in the range of 4 to 25 percent. The films were deposited in anatmosphere of up to 0.2 milliTorr of hydrogen sulphide.

[0087] The photoluminescence of the films when they were irradiated with365 nm ultraviolet light was found to increase in an approximatelylinear manner for up to 20 atomic percent europium and then decrease ata higher concentration of 25 percent europium, as shown in FIG. 4.Correspondingly, the CIE x coordinate was constant at x=0.15. The CIE ycoordinate was relatively unchanged at y=0.15 for up to 20 atomicpercent europium and increased significantly to y=0.23 at 25 atomicpercent europium.

[0088] No significant dependence of the photo-luminescence intensity orcolour coordinates on the hydrogen sulphide pressure was observed,suggesting that the films were largely saturated with sulphur.

[0089] The data indicates a change in the photoluminescence propertiesas the europium concentration is increased above 20 atomic percent,although it cannot be concluded from this example that this change isdirectly related to the increase in the europium concentration.

EXAMPLE VII

[0090] Powders to form magnesium barium thioaluminate were preparedaccording to the method used in Example II, except that the powders wereannealed at a temperature of 1000° C. under nitrogen for 10 minutes,rather than at 900° C. In addition, the fraction “a” of barium replacedby magnesium in the formula Mg_(a)Ba_(1-a)Al₂S₄:Eu was varied over therange 0.10, 0.30 0.50, 0.70 and 0.90.

[0091] Visually, the samples with a=0.5 and a 0.7 were observed to be ina liquid state following annealing, whereas the sample with a=0.3 wasobserved to be molten only at its surface. The sample with a=0.9 wasobserved to consist of at least two crystal phases that respectivelyemitted green and blue light under 365 nm ultraviolet excitation. Thesample with a=0.7 was not discernibly multi-phased.

[0092] The CIE x-coordinate for the photoluminescence was 0.14,independent of the value of “a”. The CIE y coordinate was only slightlydependent on “a”, increasing from about 0.11 for a=0.1 to 0.13 fora=0.9.

[0093] X-ray diffraction analysis of the samples showed that for thesamples with a=0.1 and a=0.3, the dominant crystal phase has a crystalstructure very close to that of barium thioaluminate and that asignificant quantity of barium sulphide was present. The ratio of thequantity of these two phases was very close to that for the samplehaving a=0.2 annealed at 900° C. in Example II (rather than 1000° C. inthe present example), indicating that the ratio of crystal phases in thepellets may be close to equilibrium for these elemental compositions.The phase having a crystal structure very close to that of bariumthioaluminate (“the barium thioaluminate-like phase”) may have a latticeconstant slightly smaller than that for pure barium thioaluminate,possibly due to the substitution of some barium in the crystal latticeby magnesium.

[0094] The sample with a=0.5 showed the barium thioaluminate-like phaseto be the dominant phase. There was no measurable quantity of bariumsulphide. This is in contrast to the sample in Example II with a=0.5that had been annealed at a lower temperature, which still showed anappreciable quantity of barium sulphide. Accordingly, for this nominalcomposition, the ratio of phases may not be an equilibrium ratio, atleast for the lower annealing temperature. The samples with a=0.5 alsohad an additional phase with a crystal structure very close to that ofmanganese thioaluminate (although this compound could not be presentbecause there was no manganese in the sample). The crystal differs fromthat of manganese thioaluminate in that an XRD peak is presentcorresponding to a crystal lattice spacing of 11.99 Angstroms thatpossibly corresponds to a superlattice distortion relative to amanganese thioaluminate structure.

[0095] The sample with a=0.7 showed the manganese thioaluminate-likephase with the 11.99 Angstrom peak as being the dominant phase, but withadditional phases present including the barium thioaluminate-like phaseand minor quantities of several phases not found in the samples withlower values of “a”. Of the samples examined, this sample appears to theonly one that is comprised largely of a single crystal structure. Anessentially single phase phosphor film in an electroluminescent displaymay be beneficial, as electrons injected into the phosphor film may havea reduced tendency to scatter from grain boundaries between differentcrystal phases and therefore have less tendency to lose energy in amanner that doesn't cause the emission of useful light. This wouldincrease the electrical-to-light energy conversion efficiency of thedisplay. It is possible that the single phase composition corresponds toa=0.75, and that the crystal structure of the compound has a crystalunit cell that has an integral multiple of the number of atoms in theunit cell for barium thioaluminate. Such a multiple may account for thelarge crystal lattice spacing of 11.99 Angstroms observed for somesamples.

[0096] The sample with a=0.9 also had the crystal phase having the 11.99Angstrom peak but substantially less of it. The sample also had theadditional phases seen in the a=0.7 sample, in greater quantity, as wellas further crystal phases not seen in the other samples. The additionalphases are consistent with the presence of a short wavelength secondaryemission peak in the PL spectra. There is also a slight shift of themain PL peak towards longer wavelength, which indicates a variability inthe atomic environment of the europium activator in the main phase asthe nominal composition is varied. This might indicate a range for theelemental composition of the main phase.

[0097] The samples with a=0.5 and a=0.7 both melted when they wereannealed at 1000° C., whereas the other samples either did not melt atall at this temperature, or only slightly melted at the surface. It maybe that a eutectic point for the compositions as a function of themagnesium to barium ratio exists for values of a between 0.4 and 0.8.From the perspective of constructing an electroluminescent display, theability to anneal the phosphor at a reduced temperature is beneficial,because minimizing processing temperatures reduces or avoids thermallyinduced damage to the display structure being formed.

[0098] The photoluminescence spectra of the samples under 365 nmexcitation was also recorded. In order to try to determine whether ornot small emission peaks were masked by the main emission peak, thespectra were de-convoluted. The center of the emission peaks wasselected. A mirror image of the half of the emission peak that decreasedmost steeply in amplitude from the center was reflected about the centreof the peak, and the thus constructed symmetrical peak was subtractedfrom the measured emission peak.

[0099] The resultant deconvoluted peak revealed the presence of smallpeaks shifted in wavelength from the main peak. The small peaks wereshifted to longer wavelength from the main peak for a=0.1, a=0.3, anda=0.5 as shown in FIGS. 5, 6 and 7 respectively. For a=0.7, theamplitude of the small secondary peak is substantially smaller, althoughit is not shifted in wavelength, as shown in FIG. 8. For a=0.9, thesecondary peak had disappeared, being replaced by another smallsecondary peak shifted to shorter wavelength from the main peak, asshown in FIG. 9. These results suggest that a minor emissive phase ispresent for “a” less than about 0.7 and that a different minor emissivephase is present for “a” approximately equal to 0.9. The larger peak wasalso observed to be shifted to longer wavelength for a=0.9, indicating achange in the crystal environment of the europium in the dominant phasecontributing to the emission at this nominal magnesium content. Thisshift is consistent with the observed increase in the CIE y coordinate.

[0100] The results obtained correlate well with the XRD results, whichindicate the presence of a barium thioaluminate-like phase for lowvalues of “a” and a multiplicity of other distinct phases for “a” near0.9. The results also correlate with the visible multi-colour lightemission from the sample with “a” about equal to 0.9 when the sample isunder ultraviolet emission.

1. A phosphor comprising a composition of the formulaM′_(a)Ba_(1-a)M″₂M′″_(4′):RE, where M′ is at least one element selectedfrom the group consisting of magnesium and calcium, M″ is at least oneelement selected from the group consisting of aluminum, gallium andindium, M′″ is at least one element selected from the group consistingof sulphur, selenium and tellurium, RE is at least one rare earthelement, and 0<a<1.
 2. The phosphor of claim 1 in which the phosphor hasbeen annealed.
 3. The phosphor of claim 1 in the form of a phosphor in athick film electroluminescent display.
 4. The phosphor of claim 3 inwhich M″ is aluminum.
 5. The phosphor of claim 4 in which RE iseuropium.
 6. The phosphor of claim 4 in which M′″ is sulphur.
 7. Thephosphor of claim 6 in which M′ is magnesium, M′″ is sulphur and RE iseuropium.
 8. The phosphor of claim 7 in which “a” is in the range of 0.4to 0.8.
 9. The phosphor of claim 7 in which “a” is in the range of 0.4to 0.8, said phosphor comprising a eutectic composition.
 10. Thephosphor of claim 7 in which “a” is in the range of 0.7 to 0.8.
 11. Thephosphor of claim 1 in the form a phosphor in a thin filmelectroluminescent display.
 12. A method for the preparation of aphosphor on a substrate, said phosphor comprising a composition of theformula M′_(a)Ba_(1-a)M″₂M′″₄:RE, where M′ is at least one elementselected from the group consisting of magnesium and calcium, M″ is atleast one element selected from the group consisting of aluminum,gallium and indium, M′″ is at least one element selected from the groupconsisting of sulphur, selenium and tellurium, RE is at least one rareearth element, and 0<a<1, said method comprising using a compound of theformula M″₂M′″₃ as a first source in a dual source electron beamevaporation apparatus and using a mixture of compounds of the formulaeM′M′″ and BaM′″ as a second source in said apparatus, said compound ofthe first source and said compounds of the second source being in theratios to provide a composition of the phosphor and at least one of thefirst and second sources including a compound of the formula REM′″, andeffecting electron beam evaporation from said first and second sourcesto a substrate to be coated with said phosphor.
 13. The method of claim12 in which the phosphor so obtained is annealed.
 14. The method claim12 in which the substrate is for a thick film electroluminescentdisplay.
 15. The method of claim 14 in which M″ is aluminum.
 16. Themethod of claim 14 in which RE is europium.
 17. The method of claim 14in which M′″ is sulphur.
 18. The method of claim 14 in which M′ ismagnesium, M′″ is sulphur and RE is europium.
 19. The method of claim 18in which “a” is in the range of 0.4 to 0.8.
 20. The method of claim 18in which “a” is in the range of 0.4 to 0.8, and the phosphor obtained iscomprised of a eutectic composition.
 21. The method of claim 18 in which“a” is in the range of 0.7 to 0.8.
 22. The method of claim 18 in whichthe phosphor is annealed at a temperature of at least about 850° C. 23.The method of claim 18 in which the phosphor is annealed at atemperature of at least about 600° C.
 24. The method of claim 18 inwhich RE is europium in an amount of not more than 3 atomic percent,based on the amount of M′ and barium.
 25. The method of claim 24 inwhich REM′″ is europium sulphide.
 26. The method of claim 25 in whichthe europium sulphide is replaced in whole or in part with europiumoxide.
 27. The method of claim 12 in which the substrate is for a thinfilm electroluminescent display.